Cultured Badass: He actually went to decent schools and got excellent grades as noted by Dignam and Queenan, he could actually have had a better career but he really wants to be a cop. He also quotes Nathaniel Hawthorne.

Deadpan Snarker: Another defense mechanism to deal with the stress of being undercover.

Didn't See That Coming: There's no way he could have anticipated being taken down like that. He called for backup by calling Brown, he mailed the evidence (making copies) and sent it to the police shrink but it takes a Giant Space Flea from Nowhere, a second mole, to take him out with no warning.

Dropped a Bridge on Him: Right as Costigan is about to arrest Sullivan and be the hero, he is shot in the head completely out of nowhere.

Expy: His undercover legend of being a former State trooper is analogous to the real-life story of Richard Marinick, a former State trooper who joined Whitey Bulger's gang.

Jerkass: What he is at heart. Though he's genuinely a decent boyfriend to Madolyn, until she finds out that he's a mole, she doesn't consider not breaking up with him, even hoping to start a family with him with a child that may or may not be his.

Karma Houdini: Actually averted. Sullivan thinks he got away from being discovered when Billy dies and frames the treason to Barrigan. Then Dignam waits for him in his apartment and kills him.

Raised Catholic: Like most Irish-American kids. The Prolonged Prologue has a shot of a young Sullivan serving as an altar boy.note Martin Scorsese stated in interviews that this was meant to imply that Sullivan might have been a victim of one of the many pedophile priests the movie makes references to.

The Starscream: He ends up betraying and killing Costello after finding out Costello's an FBI informant and was ready to expose him to the feds.

Villain Protagonist: He has a great amount of screen time and character development as Billy, yet he is one of the main villains of the film.

Wicked Cultured: As a young boy he gets a James Joyce reference and later discusses Freud seriously with a shrink. More to the point he wants to study law and apply himself and make it into Harvard, and maybe become legitimate.

A Father to His Men: He really cares about the men he oversees. When Billy goes to Queenan's home, he offers him some food and beverages to the young man.

The Good Captain: Best exemplified in a scene where he is visited by Costigan in the middle of the night to give some important info. Queenan takes him in, even gives him some food and brings him water which shows he genuinely cares for him.

Heroic Sacrifice: He lets some of Costello's boys pitch him to his death to keep Costigan's cover secure.

Reasonable Authority Figure: He is a calm fatherly man who gives some sense when tension arises and he was willing to sacrifice his own life to hide Billy's identity and the whole operation to stop Costello.

Captain George Ellerby

Colin Sullivan's superior and the one in charge for any active operation against Costello.

Beware the Nice Ones: He seems to be as reasonable as Queenan, but he can get easily excited and does not accept stupid failures easily. When the FBI didn't placed the cameras correctly in the meeting between Costello and the Chinese dealers, he almost beat the technician in charge.

Deadpan Snarker: His snarky lines provide most of the tension breakers in the film.

"I'm the guy who does his job. You must be the other guy."

Deus ex Machina: When it looks like Sullivan is about to get away with his crimes, he appears in the end to kill Sullivan in revenge.

Expy: Mark stated he was emulating every cop that had arrested him as a child as well as his own parents when they had to pay his bail with the grocery money.

Good Is Not Nice: He is a rude, obnoxious bully who needlessly antagonizes everyone he comes in contact with, but is dedicated to the job and fiercely loyal to Queenan. He saw to it that Queenan's death, as well as Costigan's (who he didn't even like) were avenged.

Costello's Gang

Francis "Frank" Costello

A ruthless and infamous old mobster targeted by the Boston Police Department.

Asshole Victim: Racist, constantly unplesant, and a mob boss. The only people sad to see him go are the FBI.

Ax-Crazy: Established early on when he and French execute a man and woman on a beach.

Costello: Jesus... she fell funny. (starts chuckling)

Mr. French: Frank, I really think you ought to see somebody.

Bad Boss: He constantly threatens to murder his underlings if they displease him. He kicks it up a notch at one point with Sullivan, threatening to have his girlfriend raped if he doesn't find the mole.

Big Bad: All these infiltration games are centred around him and his mob. Also, he corrupted a very young Colin Sullivan to join him into his mob and uses him as a mole inside the police.

The Corrupter: He tricked Colin to join his mafia when he was just a kid.

Chronic Backstabbing Disorder: Frank is betraying his own men to the FBI, taping Sullivan so he can give him up if the need arises, and screws the Chinese government over on a deal for stolen missile technology.

Dirty Old Man: In his first scene, he hits on a teenager by asking if she's had her period yet. Later, he makes a pass at a nun.

The Don: Has been the head of his own gang for decades, and is the most wanted organized crime figure in Boston.

Evil Old Folks: Age has not slowed down his rampant criminal activities, nor has it made him any less intimidating.

Faux Affably Evil: He comes across as a charismatic guy for at most, a few seconds, before revealing what a depraved psychotic he truly is.

For the Evulz: The reason he truly does what he does. At his core, Frank is a sadist.

Billy: You don't need the money.

Costello: I haven't "needed the money" since I took Archie's milk money in the third grade. Tell you the truth, I don't need pussy any more either... but I like it.

Hypocrite: Costello is homicidally obsessed with snuffing out the rat in his crew. This makes it all the more ironic when he is revealed to be an informant for the FBI, selling out every member of his crew, including his informants in the police like Sullivan.

Genre Savvy: Costello is aware that sending The Mole into the police might lead him to become a double agent, so he sends another mole, Barrigan, to keep an eye on Sullivan.

The Informant: He is actually informing the FBI and it's the reason why the Boston police can't arrest him for his crimes.

It's All About Me: It becomes clear that Frank Costello only has loyalty to himself alone. This is proven when he's revealed to be an FBI mole compiling information on every one of his crew, including Colin Sullivan who he acts as a Parental Substitute to. And taken further when he quickly leaves his trusted Number Two Mr. French to die in a car fire during the sting operation.

Large Ham: Jack Nicholson explodes any piece of charisma and presence to his performance and chews almost every scenery like a rat.

No Historical Figures Were Harmed: His character is heavily inspired by Boston gangster, Whitey Bulger, who both had an informant in the FBI and was an informant himself.

Politically Incorrect Villain: Some of his snarky comments are filled with lots and lots of black humor, whose targets includes black people, Asians and pedophile priests. Also, he slept with a nun and flirts with a teenage girl.

Properly Paranoid: For as crazy as he is about it, he's not wrong about there being a cop in his crew.

Sanity Slippage: As Costigan tells Queenan, Costello seems to get crazier as the rumor of the mole is intensifying. One scene involves Costello coming to the pub's bar with his arms covered in blood and acting like normal, telling his men to clean the mess he left.

The Sociopath: Murders, tortures and stabs people in the back without a care in the world. At one point, he casually pulls out a severed hand in a plastic bag while having a private chat with Costigan over lunch.

Timothy Delahunt

Ambiguous Situation: After his body is found, the news reports that he was an undercover cop who infiltrated Costello's gang. Fitzy and the others buy into this, but Costello is convinced that he was labelled as such so that he will think there is no longer a rat in the crew. Which makes sense, since there is still a rat in the crew: Costigan. It's never stated which is correct.

Hidden Depths: Revealed as he's dying, when he speaks to Costigan and is later revealed to be an undercover cop.

I Never Said It Was Poison / Dying as Yourself: Delahunt figures out that Costigan is an undercover cop because he is at the place where the other gangsters were congregating to catch the mole, even though Costigan was accidentally given the wrong address.

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